Calibration circuit

ABSTRACT

An apparatus is provided to calibrate a target electrical circuit. The target electrical circuit includes at least a first variable capacitor and a first resistive element. The apparatus includes a second variable capacitor proportionally matched to the first variable capacitor. The apparatus also includes a measurement branch having at least a second resistive element proportionally matched to the first resistive element, and a current generator to generate a current proportionally matched to a predicted current of the target circuit. The apparatus also includes a digital loop to generate a digital code based on at least a comparison between a first voltage signal of the measurement branch and a predetermined voltage.

CONTINUING DATA

The present application is a Divisional of prior application Ser. No. 09/629,095, filed on Jul. 31, 2000, and issued on Jun. 10. 2003 as U.S. Pat. No. 6,577,114, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to calibrating electrical circuits, such as signal filters. In particular, the present invention relates to an electrical calibration circuit.

2. Background and Related Art

Electrical circuits are often fabricated by processes that introduce unexpected or unwanted characteristics. Such characteristics may vary a circuit's performance from its designed specifications. External factors such as temperature can also vary the performance of a circuit.

A Sallen and Key (“SK”) low pass filter is depicted schematically in FIG. 1. As will be appreciated by those skilled in the art, the illustrated SK filter provides a second-order response. As shown in FIG. 1, the SK filter includes a known 1×buffer 1, resistors R₁ and R₂, and capacitors C₁ and C₂. A straightforward analysis of the SK filter yields the following transfer function, with R₂=R₁: $\begin{matrix} {{{H(s)} = \frac{1}{S^{2} + {\frac{Wp}{Q}S} + {Wp}^{2}}},} & (1) \end{matrix}$

where $\begin{matrix} {{{Wp} = \frac{1}{\sqrt{R_{1}R_{1}C_{1}C_{2}}}},\quad {and}} & (2) \\ {\frac{1}{Q} = {2{\sqrt{\frac{R_{1}C_{2}}{R_{1}C_{1}} = {2\sqrt{\frac{C_{2}}{C_{1}}}}}.}}} & (3) \end{matrix}$

SK Filters are often employed in circuits that require precise signal responses. However, such filters are rendered ineffective if their characteristics, such as a cutoff frequency (ω_(c)), are altered in the fabrication process. For example, a design may call for a 10 pF capacitor, but the fabricated capacitor may actually have a capacitance of 11.5 pF. Process variance is particularly problematic for integrated circuits, since integrated components are not easily calibrated.

Conventional calibration techniques, such as calibrating for a precise resistance or capacitance, are insufficient because both resistance and capacitance can vary, and the cutoff frequency (ω_(c)) is a function of the RC product.

Hence, there are many accuracy-related problems to be solved in the art. Accordingly, an electrical calibration circuit is needed in such circuits. There is a further need for an electrical calibration circuit to calibrate signal filters. There is also a need for a calibration method to correct for fabrication variance.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the drawbacks noted above and to provide an effective electrical calibration circuit. The calibration circuit can be used to calibrate integrated electrical circuits, such as filters.

According to a first aspect of the present invention, an apparatus to calibrate a target electrical circuit is provided. The target electrical circuit includes at least a first variable capacitor and a first resistive element. The apparatus includes a second variable capacitor proportionally matched to the first variable capacitor, and a measurement branch. The measurement branch has at least a second resistive element proportionally matched to the first resistive element, and a current generator to generate a current proportionally matched to a predicted current of the target circuit. The apparatus also includes a digital loop to generate a digital code based on at least a comparison between a first voltage signal of the measurement branch and a predetermined voltage.

According to another aspect of the present invention, a method of calibrating a target electrical circuit is provided. The target electrical circuit includes at least a first variable capacitor and a first resistive element. The method includes the steps of: i) matching a second variable capacitor to the first variable capacitor; ii) providing a measurement branch having at least a second resistive element proportionally matched to the first resistive element, and providing a current generator to generate a current proportionally matched to a predicted current of the target circuit; and iii) generating a digital code based on at least a comparison between a first voltage signal of the measurement branch and a predetermined voltage.

According to still another aspect of the present invention, a method of calibrating a target circuit with a calibration circuit is provided. The target, circuit includes at least a first and second variable capacitor, and the calibration circuit includes at least a third variable capacitor. The method includes the steps of: i) proportionally matching components in the calibration circuit with the target circuit; ii) generating a digital code in response to a voltage change in the calibration circuit; and iii) providing the digital code to each of the three variable capacitors.

According to yet another aspect of the present, invention, a calibration circuit is provided. The calibration circuit includes a first capacitor having a plurality of parallel capacitors. At least one parallel capacitor of the plurality of parallel capacitors communicates with a switch, and the switch is operated with a digital signal. The a calibration circuit includes a digital loop having an output in communication with at least the switch of the one parallel capacitor. The digital loop generates a digital signal and communicates the digital signal to at least the switch. The digital signal is varied to reach a steady-state for the calibration circuit.

These and other objects, features and advantages will be apparent from the following description of the preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more readily understood from a detailed description of the preferred embodiments taken in conjunction with the following figures.

FIG. 1 is a circuit diagram illustrating a conventional Sallen and Key low pass filter.

FIG. 2 is a circuit diagram of a third-order Sallen and Key low pass filter.

FIG. 3 is a circuit diagram of a calibration circuit according to the present invention.

FIG. 4 is a timing chart of clocks φ₁ and φ₂.

FIG. 5a is a circuit diagram illustrating a switch response of the FIG. 3 calibration circuit at a time t₁ as shown in FIG. 4.

FIG. 5b is a circuit diagram illustrating a switch response of the FIG. 3 calibration circuit at a time t₂ as shown in FIG. 4.

FIG. 5c is a circuit diagram illustrating a switch response of the FIG. 3 calibration circuit at a time t₃ as shown in FIG. 4.

FIG. 6 is a circuit diagram illustrating a structure of a capacitor C_(i).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will be described with respect to a calibration circuit for calibrating a low pass filter. However, as will be appreciated by those skilled in the art, the inventive calibration circuit is also applicable to calibrating other RC circuits, such as other filters, electronic circuits, and so forth.

FIG. 2 illustrates a third-order, low pass (cascaded) filter 2 arranged in an SK configuration. As will be appreciated, a third order response is achieved by cascading a first-order filter (e.g., R₃ and C₃) with a second-order SK filter. Such a filter could be used, for example, to filter a receive signal in an Ethernet gigabit channel, as described in a co-pending application by the same inventor of this application, titled “ACTIVE RESISTIVE SUMMER FOR A TRANSFORMER HYBRID,” filed on 31-Jul.-2000 and assigned Ser. No. 09/629092, and hereby incorporated by reference. Of course, the illustrated filter 2 could be used in other applications as well.

The low pass filter 2 preferably includes a variable capacitor C₁, capacitors C₂ and C₃, and resistors R₁, R₂, and R₃. Capacitors C₂ and C₃ have equal capacitance in the preferred embodiment. Similarly, resistors R₁, R₂, and R₃ are equal (e.g., R₁=R₂=R₃) in the preferred embodiment. Of course, the capacitance of C₂ and C₃ respectively, as well as resistors R₁, R₂, and R₃, respectively, can be varied according to need or filter design. In an alternative arrangement, low pass filter 2 includes variable capacitors C₁, C₂, and C₃. A preferred structure for the variable capacitors is shown in FIG. 6, and is discussed in even greater detail below. Varying the capacitance of C₁ calibrates the low pass filter 2. In the alternative arrangement, the capacitance of C₁, C₂ and/or C₃ can be varied to calibrate filter 2. For the second-order SK stage of filter 2, Q is preferably equal to one (1), which yields C₁=4C₂, as shown by Equation 3. Preferred parameters for the SK filter 2 are shown in Table 1.

TABLE 1 Preferred Low pass Filter Parameters Component Preferred Value R₁ = R₂ = R₃ 10K Ohms C₁ 800 fF C₂ = C₃ 200 fF DC Gain 1 Cutoff frequency 50 Mhz Buffer Characteristics Bandwidth = 400 MHz

Due to fabrication or external factors, the SK filter 2 may have undesirable filtering characteristics. Calibration of the circuit will enhance the filter 2's performance and/or achieve designed specifications. Accordingly, a calibration circuit and method will be described in further detail below.

A relationship between a target circuit (e.g., the filter circuit to be calibrated) and a calibration circuit 3 is found. It will be appreciated by those skilled in the art that the current I through the C₂ capacitor (as shown in FIG. 2) is defined as follows: $\begin{matrix} {{I = {\frac{q}{t} = \frac{C_{1}\Delta \quad V_{C1}}{\Delta \quad t}}},} & (4) \end{matrix}$

where q is charge, ΔV_(c1) is the change in voltage across the C₁ capacitor, and Δt is the amount of charging time. If the current I (Equation 4) flows through a measurement branch of a calibration circuit, then a voltage (V_(R)) in a measurement branch of the calibration circuit can be expressed as:

V _(R) =K ₁ IR,  (5)

where K₁ is an arbitrary constant and R represents R₁, if R₁=R₂ =R₃. Of course, the inventive techniques and resulting structures can. also be achieved when R₁, R₂, and R₃ are not equal, with straightforward modifications that will be appreciated by those skilled in the art.

Substituting Equation 4 into Equation 5 yields Equation 6, below: $\begin{matrix} {V_{R} = {K_{1}{\frac{C_{1}\Delta \quad V_{C1}R}{\Delta \quad t}.}}} & (6) \end{matrix}$

A constant K₃(where K₂=K₁/Δt) simplifies Equation 7 as follows:

V _(R) =K ₂ΔV_(c1) C ₁ R.  (7)

If V_(R)=ΔV_(c1), then: $\begin{matrix} {{\frac{1}{K_{2}} = {C_{1}R}},} & (8) \end{matrix}$

Selecting values for C₁ and R, based on a desired cutoff frequency, yields a value for K₂. A value for K₁ can then be found since K₂=K₁/Δt. In the preferred embodiment K₁ equals 15.

K₁ can be mapped onto the calibration circuit 3 by attributing its value to different components as follows:

K ₁=15=(C) (I) (R)=(2) (3) (2.5),  (9)

where C corresponds to C₂ in FIG. 2, I is a measurement current and R is the selected resistor. These values can then be mapped onto components to construct a calibration circuit 3 shown in FIG. 3. Of course, K₁ could be apportioned in different amounts or to different components, and the corresponding amounts and components could be adjusted accordingly.

Calibration circuit 3 (as shown in FIG. 3) and low pass filter 2 (as shown in FIG. 2) are preferably designed and constructed via customized CMOS integrated circuits. It is even more preferably to fabricate circuits 2 and 3 near in time, such as during the same process run (e.g., preferably on the same chip and near the same chip location). In this manner, the process tolerances and fabrication conditions will be nearly identical for each of the calibration and target (e.g., filter 2) circuit. Accordingly, component characteristics and values, such as resistor and capacitor values, can be closely matched between the two circuits 2 and 3.

A preferred calibration circuit 3 configuration is shown in FIG. 3. A known current source 10, providing a current I, is placed in series with a transistor 11. Transistor 11 can be a known transistor, such as a CMOS n-channel transistor. The output of an integrator 12 is applied to the gate 11 a of the transistor 11. The integrator 12 can be constructed in a known manner. For example, an op-amp is preferably arranged in an inverting configuration by placing a capacitor C between the output terminal and the inverting (e.g., negative) terminal of the op amp. In a preferred embodiment, a capacitance for C is selected to be about 50 times a capacitor, C₄ to obtain an integration factor of 1/(50T), with a clock period (T) of about 64 ns. Of course, a different value for C could be selected according to need, clock speed, integration characteristics, and so forth.

A switch S2 is placed in communication with the integrator 12 (e.g., in communication with the negative terminal of the op-amp). Switch S2 switches a connection with the capacitor C₄. As shown in FIG. 3, a switch S3 gates a parallel arrangement between the C₄ capacitor and a variable capacitor C₅. A preferred structure for the C₅ variable capacitor is shown in FIG. 6.

A reference voltage V_(ref) is input to the positive terminal of the op amp. V_(ref) will be described in even further detail below. The current I is incrementally adjusted as a result of the varying voltage at the gate 11 a of the transistor 11. The current I stabilizes when the voltage across the C₄ capacitor, at the end of each charging event, equals V_(ref) (e.g., when the output of the integrator stabilizes or becomes constant).

A known current mirror 13 is arranged to be in communication with the C₅ capacitor, a switch S1, and the S3 switch, as shown in FIG. 3. Current mirror 13 is preferably a 1× mirror (e.g., one times a current source) that mirrors current I provided by current source 10.

The switch S1 is arranged in parallel with the C, capacitor. As will be appreciated, NMOStransistors can be used as switches S1, S2 and S3. A known 3× current mirror 14 mirrors current source 10 to provide a, current I_(R), which is, e.g., preferably equal to 3·I. The 3× mirror accommodates the “3·I” requirement established by Equation 9. A voltage V_(r2) is measured across resistors R₄ and R₅. Similarly; a voltage V_(r1) is measured across the resistor R₅. Resistor R₅, as shown in FIG. 3, preferably has a resistance of “2.5·R₁,” if R₁=R₂=R₃, which accommodates the resistive requirement established by Equation 9. Also, C₅ (FIG. 3) preferably equals 2·C₂ (FIG. 2) which accommodates the capacitance requirement as established by Equation 9.

A digital loop 15 provides C₆ with a digital code to vary its capacitance. The digital loop 15 preferably includes a plurality of shift registers or an updown counter with appropriate logic, for example. A digital comparator 16 makes a comparison between V_(r1) and V_(ref). The output from comparator 16 is provided to the digital loop to shift the loop up or down. A digital comparator 17 makes a comparison between V_(r2) and V_(ref). The output from comparator 17 is also provided to the digital loop.

The calibration circuit 3 continues to adjust the capacitance of C₅ until circuit 3 stabilizes. A stable circuit 3 is achieved when the current I_(R) reaches a steady state. At this steady state point, V_(r1) approximately equals V_(ref). Preferably, the C₁ capacitor in the filter 2 is proportionally adjusted in response to the value of the C₅ capacitor, which caused the calibration circuit 3 to reach its steady-state, thus calibrating the filter 2. A preferred method and apparatus for adjusting the C₁ and C₅ capacitors are further discussed below. Preferred component and parameter values for the calibration circuit 3 are shown in Table 2, below.

TABLE 2 Preferred Calibration Circuit Parameters Component Preferred Value R₄ 500 Ohms R₅ 25K Ohms C₄  10 fF C₅ 400 fF C 500 fF Transistor 11 NMOS L = 1.8 μm, W = 6 μm Comparators 16 and 17 Offset · 10 mV Setting time · 10 μs Current I 40 μA V_(ref) 1.5 volts

Operational aspects of the calibration circuit will be further described with reference to FIGS. 4, 5 a, 5 b and 5 c. A 125 MHz system clock (not shown) is preferably divided by eight (8) in a known manner to produce at least three (3) 15.625 MHz clock signals φ₀, φ₂ and φ₂. Of course, a different system clock in combination with different component values could be used to achieve similar results. Clock φ₀ preferably drives (e.g., clocks) the digital loop 15.

A timing chart of clocks φ₁ and φ₂ is shown in FIG. 4. As will be readily appreciated, a 15.625 MHz clock has a 64-nanosecond period. As shown, φ₂ is inverted in a known manner from the φ₁ signal. Clocks φ₀, and 2 drive switches S1, S2 and S3 as shown in Table 3, below.

TABLE 3 Switch Assignments Clock Drives Switch φ₁ S₁ φ₁ S₂ φ₂ S₃

In time period t₁ (shown in FIG. 4), φ₁ is high and φ₂ is low Accordingly, switches S1 and S2 are closed while switch S3 is open, as shown in FIG. 5a. The voltage across C₅ is effectively drained during period t₁ by the short created through S1.

In time period t₂ (shown in FIG. 4), φ₁ is low and φ₂ is high. Accordingly, switches S1 and S2 are open while switch S3 is closed, as shown in FIG. 5b. During the t₂ period, C₄ and C₅ are in parallel and are each charged by the constant current I. C₄ should be relatively small compared to C₅, so that C₅ does not glitch C₅ (e.g., cause a large charge step) when S3 closes. As will be appreciated, the final voltage across C₄ and C₅ can be determined from the capacitance, constant, current I, and charging time (e.g., 32 nanoseconds).

The φ₁ clock is high and the φ₂ clock is low in time period t₃. In the t₃ period, as shown in FIG. 5c, switches S1 and S2 are closed while switch S3 is open. The short created by switch S1 effectively drains the voltage across C₅. The voltage across C₄ incrementally drives the integrator 12 during period t₃. The output of the integrator 12 is applied to gate 11 a of the transistor 11, which causes an incremental change in the current I flowing through the transistor 11. As will be appreciated, the transconductance (ΔI/ΔV) of the transistor 11 sets the change in current I. This incremental change in current I is reflected to the current mirror 14. Accordingly, a corresponding incremental change in I_(R) is produced (e.g., three (3) times the change to current I). The voltages V_(r2) and V_(r1) are also incrementally varied.

V_(r1) is compared against V_(ref) by the comparator 16. If V_(r1)>V_(ref), the digital comparator outputs a digital 1. Such an output indicates that the capacitance of C₅ should be increased. When V_(r1)>V_(ref), current I is too big (according to Equation 4) and, therefore, increasing the capacitance causes current I to decrease. Accordingly, the shift registers in the digital loop 15 are shifted up, thus incrementing a “digital code” generated by the registers. The digital comparator 16 outputs a digital if V_(r1)<V_(ref). Such an output indicates that the capacitance of C₅ should be decreased. When V_(r1)<V_(ref), current I is too small (according to Equation 4) and, therefore, decreasing the capacitance of C₅ causes current I to increase. The digital loop 15 shifts the registers down in response to a zero (0) from comparator 16, thus adjusting down the digital code. As will be appreciated, V_(ref) is canceled by letting V_(R)=ΔV_(c1) as shown in Equation 7. Accordingly, an arbitrary value can be selected as the reference voltage V_(ref). However, V_(ref) is preferably at least equal to the bias voltage of the integrator op amp.

Voltage V_(r2) is also compared against voltage V_(ref) by the comparator 17. This comparison functions as a threshold to prevent the calibration of C₅ due to only minor changes in the current I_(R). For example, the output of comparator 17 enables the registers to accept input from comparator 16 when V_(r2)>V_(ref). In a preferred embodiment, R₄ is chosen such that V(r₂)−V(r₁) is about 60 mV.

The output of the digital loop 15 (e.g., the digital code) calibrates the capacitance of C₅. In FIG. 6, C_(i) is a variable capacitor, where i corresponds to a respective capacitor. For example, when discussing the preferred structure of C₅, i equals “5.” With reference to FIG. 6, C_(“5”) includes a plurality of parallel capacitors C_(5-b) and C₅₋₁ through C₅₋₂₀. Capacitors C₅₋₁ through C₅₋₂₀ are each communicating in series with a switch S_(i-1) through S_(i-20), respectively. Switches S_(i-1) through S_(i-20) can be realized with a CMOS n-channel transistor, for example, or other known switching components. Capacitors C₅₋₁ through C₅₋₂₀ preferably have the same capacitance (e.g., C₅₋₁=C₅₋₂=C₅₋₃, and so on). Incremental calibration steps can be made with these equivalent capacitors. Capacitor C_(5-b) establishes a base capacitance for C₅. As such, C_(5-b) is preferably not switched.

The digital loop 15 calibrates C₅ by providing a digital code to the parallel C₅ arrangement. The digital code contains a binary digit for each switch S_(i-1) through S_(i-20) (e.g., switches S₅₋₁ through S₅₋₂₀, with respect to the C₅ capacitor). On an initial calibration cycle, the digital loop supplies (e.g., communicates) a code of 00 . . . 01 if V_(r1)>V_(ref) (when the digital loop is enabled by the V_(r2) comparison). Only switch S₅₋₂₀ is closed with this code. Accordingly, the equivalent C₅ capacitance is C_(5-b)+C₅₋₂₀. If the digital loop is subsequently incremented due to the comparison of V_(r1) and V_(r2) (each against V_(ref)) a digital code of 000. . . 11 is supplied to C₅. Switches S₅₋₁₉ (not shown) and S₅₋₂₀ are closed by this code. Accordingly, the equivalent C5 capacitance is C_(5-b)+C₅₋₁₉+C₅₋₂₀. Capacitors C₅₋₁ through C_(5-n) can be incrementally added in or subtracted from the equivalent C₅ capacitance based on the digital code to achieve stability of the calibration circuit 3. The maximum capacitance of the illustrated C₅ circuit is C_(5-b)+Σ(C₅₋₁ through C₅₋₂₀). Of course, a different number of parallel capacitors could be used according to the designed level of calibration. Also, the digital code could be generated and supplied in an alternative sequence, e.g.,

100 . . . 00, 110 . . . 00, etc.

Capacitance values for C₅ are found to stabilize the calibration circuit 3 . These values correspond to the digital code (e.g., a stabilizing, code) provided by the digital loop 15. Preferred values for the parallel C₅ arrangement are shown in Table 4, below.

TABLE 4 Preferred Values for C₅ Component Value C_(5-b) 240 fF C₅₋₁ -C₅₋₂₀  16 fF

In this preferred embodiment, the C₅ capacitance: is varied in increments of 16 fF. The nominal capacitance of 400 fF is achieved when ten capacitors (e.g., C₅₋₁₁ through C₅₋₂₀, each having a 16 fF capacitance) are included with the base capacitor C_(5-b) (240 fF). In the preferred embodiment, ten additional capacitors (e.g., C₅₋₁ through C₅₋₁₀) can also be included if needed to reach a steady state. Of course, parallel capacitors can also be subtracted from the total C₅ capacitance when needed.

Preferably, C₁ is constructed in the manner shown in FIG. 6. In the preferred embodiment, C₁ includes a base capacitor C_(1-b), which is equal to 480 fF, and a plurality of parallel capacitors C₁₋₁ through C₁₋₂₀, which are each equal to 32 fF. The nominal capacitance of 800 fF is achieved when ten capacitors are included with the base capacitor C_(1-b).

The digital code is concurrently supplied to the C₁ capacitor of circuit 2 and the C₅ capacitor in circuit 3 . Accordingly, the filter 2 is calibrated based on the stabilizing digital code of the calibration circuit 3.

In an alternative arrangement, the digital code is also supplied to the C₂ and C₃ capacitors. With this arrangement, the C₂ and C₃ capacitors are proportionally constructed with the format shown in FIG. 6. Preferably, in this alternative arrangement, the base capacitors (e.g., C_(2-b) and C_(3-b)) each equal 120 fF and the switched, parallel capacitors (C₂₋₁ through C₂₋₂₀ and C₃₋₁ through C₃₋₂₀) each equal 8 fF.

Of course, a new digital code can be automatically, supplied to the filter 2 when the calibration circuit 3 determines that a new code is needed. For example, a change in temperature may cause a shift in current from an initial steady state. Hence, the calibration circuit 3 adjusts for process variance, as well as environmental changes.

Hence, an electronic calibration circuit and corresponding calibration methods have been described in detail herein.

While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it will be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention covers various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

For example, while a calibration circuit and low-pass filter are preferably constructed using integrated circuits, it will be appreciated by those skilled in the art that the inventive circuits and corresponding configurations could also be realized using discrete components.

Also, while preferred circuit configurations and component values have been described, those skilled in the art will appreciate that modifications could be made without deviating from the inventive methods and structures. For example, resistor and capacitor values could be changed according to need, circuit design, signal responses, and so forth. Also, the resistors R₁, R₂, and R₃ (as well as capacitors C₂ and C₃) can be varied with respect to each other. Corresponding components in the target circuit and calibration circuit 3 could be adjusted to reflect these types of changes. These and other such modifications are covered by the present invention.

As a further modification, the digital code generated by the digital loop 15 could be provided to a look-up table. The digital code could be used to index respective corresponding values (e.g., codes) for the variable C₁, C₂, C₃, and C₅ capacitors. The corresponding look-up table codes could be communicated to the corresponding variable capacitors. This arrangement may be particularly useful when non-linear relationships exist between the variable capacitors.

As an additional modification, some or all of the resistors (FIGS. 2 and 3) could include a variable resistance. These variable resistors could also include a different resistance with respect to each other. The variable resistors preferably include a resistive arrangement with switches. The digital code is then provided to the variable resistors, e.g., resistor R₅, for circuit calibration. In one embodiment, the digital code is first provided to the variable resistors R₅ and resistors R₁, R₂, and/or R₃ for calibration, and then the code is provided to the variable capacitors for finer calibration. The dashed line in FIG. 3 illustrates an embodiment where the digital code is provided to a variable R₅ resistor.

Also, whereas a filter has been shown as the target circuit, other target circuits could be calibrated as well, such as electrical circuits containing RC components, where the target design parameters depend on the RC product. 

What is claimed is:
 1. An electrical circuit comprising: a filter including at least a first variable capacitor and a second variable capacitor and at least a first and second resistor; and a calibration circuit comprising: a current source; a transistor communicating in series with said current source, the transistor including a gate; an integrator communicating with the gate of the transistor; a third capacitor; a first switch communicating between the integrator and the third capacitor; a fourth variable capacitor; a second switch communicating between the third capacitor and the fourth variable capacitor, the second switch having a closed position, wherein the third capacitor and the fourth variable capacitor are in a parallel arrangement when the second switch is in the closed position; a third switch in parallel with the fourth variable capacitor; a first current mirror communicating with each of the third switch, the fourth variable capacitor and the second switch, wherein the first current mirror mirrors the current source; a second current mirror which proportionally mirrors the current source; a third resistive element in series with the second current mirror; and a digital loop communicating with each of the first, second, and fourth variable capacitors.
 2. The electrical circuit according to claim 1, further comprising a first comparator having an output in communication with the digital loop, the first comparator comparing a voltage across the third resistive element with a predetermined voltage.
 3. The electrical circuit according to claim 2, further comprising a fourth resistive element in communication between the second current mirror and the third resistive element, and a second comparator having an output in communication with the digital loop, wherein the second comparator compares a second voltage across the third resistive element and the fourth resistive element with a predetermined voltage.
 4. The electrical circuit according to claim 3, wherein the digital loop generates a digital signal in response to the output of the first comparator and the second comparator.
 5. The electrical circuit according to claim 4, wherein the digital signal is communicated to each of the three variable capacitors.
 6. The electrical circuit according to claim 5, further comprising a first clock and a second clock.
 7. The electrical circuit according to claim 6, wherein the first clock drives the first switch and the third switch and the second clock drives the second switch.
 8. An electrical circuit comprising: means for filtering including at least first means for providing varying capacitance, second means for providing varying capacitance, and at least first means for providing varying resistance and second means for providing varying resistance; and means for calibrating including: means for providing current; means for switching in series with said means for providing current, the switching means including means for gating; means for integrating in communication with the gating means; third means for providing capacitance; first switching means for switching in communication between the integrating means and the third means for providing capacitance; fourth means for providing variable capacitance; second switching means for switching in communication between the third means for providing capacitance and the fourth means for providing variable capacitance, the second switching means having a closed position, wherein the third capacitance means and the fourth variable capacitance means are in a parallel arrangement when the second switching means is in the closed position; third switching means for switching that is in parallel with the fourth variable capacitance means; first means for mirroring current in communication with each of the third switching means, the fourth variable capacitance means, and the second switching means, wherein the first mirroring means mirrors the means for providing current; second means for mirroring which proportionally mirrors the means for providing current; means for providing resistance in series with the second mirroring means; and means for providing a digital code that communicates with at least each of the first, second, and fourth variable capacitance means.
 9. The electrical circuit according to claim 8, further comprising a first means for comparing having an output in communication with the means for providing a digital code, the first comparing means comparing a voltage across the third resistance: means with a predetermined voltage.
 10. The electrical circuit according to claim 9, further comprising a fourth means for providing resistance communicating between the second mirroring means and the third resistance means, and a second means for comparing having an output in communication with the providing a digital code means, wherein the second comparing means compares a second voltage across the third resistance means and the fourth resistance element with a predetermined voltage.
 11. The electrical circuit according to claim 10, wherein the digital code means generates a digital signal in response to the output of the first comparing means and the second comparing means.
 12. The electrical circuit according to claim 11, wherein the digital signal is communicated to each of the first, second and fourth variable capacitance means.
 13. The electrical circuit according to claim 8, further comprising first means for clocking and second means for clocking.
 14. The electrical circuit according to claim 13, wherein the first clocking means clocks the first switching means and the third switching means and the second clocking means clocks the second switching means.
 15. An integrated circuit comprising: a first circuit to be calibrated and having a resistance and a first variable capacitance; a second calibrating circuit having a second resistance and a second variable capacitance, said second circuit being disposed on the same substrate as said first circuit; and circuitry concurrently producing a digital code to the first and second: variable capacitance to calibrate said first circuit.
 16. An integrated circuit comprising: a first circuit to be calibrated and having a resistance and a first variable capacitance; a second calibrating circuit having a second resistance and a second variable capacitance, said second circuit being disposed on the same substrate as said first circuit; and means for concurrently producing a digital code to the first and second variable capacitance to calibrate said first circuit. 